Hi Eliot (and List):
At the level of coding by single neurons, there is a significant
literature on sensitivity to frequency (as opposed to pitch) change
and to direction of frequency change.
The issue is complicated by the fact that the neural mechanisms
engaged by the stimulus, and therefore the direction preference
expressed in the neural responses, may depend on the form that the
frequency modulation (FM) takes. Some authors have used sinusoidal
(i.e., periodic) FMs. Others have used transient (often linear) FMs,
either in the form of an FM tone pulse, or in the form of a transient
FM of an otherwise continuous tone. As you might imagine, the
constellations of adaptive, inhibitory and other mechanisms triggered
by these stimuli could be very different. Indeed, my bet is that a
neuron could reverse its FM direction preference depending on whether
the FM was presented as a tone pulse or as a modulation of an ongoing
carrier. Of course, as is often the case, the ambiguity in the
response output of any single neuron is likely resolved by the
ensemble response.
The following is a small sample of cortical single-neuron studies on
FM coding. It will give you a sense of the ways in which some
single-unit people have thought about it.
I hope that this helps. All good wishes,
Dennis Phillips
Suga, N. (1965) Analysis of frequency-modulated sounds by auditory
neurones of echo-locating bats. J. Physiol., 179: 26-53.
Phillips, D.P. (1988) The neural coding of simple and complex
sounds in the auditory cortex. In J.S. Lund (Ed.), Sensory
Processing in the Mammalian Brain: Neural Substrates and Experimental
Strategies. NY: Oxford University Press, 172-203.
Heil, P., Rajan, R. and Irvine, D.R.F. (1992) Sensitivity of
neurons in cat primary auditory cortex to tones and
frequency-modulated stimuli. I: Effects of variation of stimulus
parameters. Hearing Res., 63: 108-134.
Eggermont, J.J. (1994) Temporal modulation transfer functions for
AM and FM stimuli in cat auditory cortex. Effects of carrier type,
modulating waveform and intensity. Hearing Res., 74: 51-66.
Tian, B. and Rauschecker, J.P. (1998) Processing of
frequency-modulated sounds in the cat's posterior auditory field. J.
Neurophysiol., 79: 2629-2642.